1. Field of the Invention
The present invention relates to white balance processing for an image pickup device such as a digital camera or a video camera.
2. Related Background Art
White balance processing in a conventional image pickup device is conducted as follows.
First, a signal that is outputted from an image device through primary-color filters is digitalized by A/D conversion, and divided into blocks shown in FIG. 1A, respectively. Each of the blocks is structured by a unit including respective color signals R, G1, G2, and B, one by one, as shown in FIG. 1B. A color evaluated value of each of those blocks is calculated (where Cx=(R−B)/Y and Cy=(R+B−2)/Y).
                                                                        C                x                            =                                                                    (                                          R                      +                                              G                        2                                                              )                                    -                                      (                                          B                      +                                              G                        1                                                              )                                                                    Y                  i                                                                                                                        C                y                            =                                                                                          (                                              R                        +                        B                                            )                                        ⁢                                          /                                        ⁢                    4                                    -                                                            (                                                                        G                          1                                                -                                                  G                          2                                                                    )                                        ⁢                                          /                                        ⁢                    4                                                                    Y                  i                                                                                                                        Y                i                            =                                                R                  +                                      G                    1                                    +                                      G                    2                                    +                  B                                4                                                                        Expression        ⁢                                  ⁢                  (          1          )                    
FIG. 2 shows a color space in which the axis of abscissa is Cx=(R−B)/Y and the axis of ordinate is Cy=(R+B−2)/Y, and shows a white axis determined by picking up a white color at a high color temperature to a low color temperature in advance and plotting color evaluated values Cx and Cy. Because there exists a slight variation of white color in an actual light source, it is assumed that a wide area having the white axis as a center is set as a white detection area (an area to be judged as white). That is, the color evaluated values Cx and Cy obtained in the respective blocks are plotted in the color axis shown in FIG. 2. Then, it is assumed that a block of the color evaluated values included in the white detection area among all of the color evaluated values is white color. In addition, integrated value SumR, SumG1, SumG2, and SumB in the color pixels within the white detection area are calculated to calculate a white balance gain by using the following expressions. In the expressions, kWB_R, kWB_G1, kWB_G2, and kWB_B are white balance gains of the respective color signals R, G1, G2, and B.kWB—R=1.0/SumR kWB—G1=1.0/SumG1 kWB—G2=1.0/SumG2 kWB—B=1.0/SumB  Expression (2)
However, the conventional white balance coefficient calculation suffers from the following drawbacks. At high color temperature, the white color evaluated values distribute in the vicinity of an area A in FIG. 2. However, under a high color temperature light source, if, for example, the color evaluated values Cx and Cy of a skin are plotted in the color axis, the color evaluated values Cx and Cy are caused to distribute at the low color temperature side in the white detection area. Therefore, in a scene where a white color is little in a photography screen and a close-up of the skin is taken, the color evaluated value of the screen is caused to distribute in an area B shown in FIG. 2. That is, there arises such a problem in that the skin color is erroneously judged as the white color at low color temperature, and the skin is colored with white color.